The 2–8 keV cosmic X-ray background spectrum as observed with XMM-Newton ^*,**

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Milano “G. Occhialini” – CNR v.Bassini 15, 20133 Milano, Italy
² Università di Milano Bicocca, Dipartimento di Fisica, P.za della Scienza 3, 20126 Milano, Italy

Corresponding author: A. De Luca, deluca@mi.iasf.cnr.it

Received: 30 September 2003
Accepted: 17 November 2003

Abstract

We have measured the spectrum of the Cosmic X-ray Background (CXB) in the 2–8 keV range with the high throughput EPIC/MOS instrument onboard XMM-Newton. A large sample of high galactic latitude observations was used, covering a total solid angle of 5.5 square degrees. Our study is based on a very careful characterization and subtraction of the instrumental background, which is crucial for a robust measurement of the faintest diffuse source of the X-ray sky. The CXB spectrum is consistent with a power law having a photon index  ± 0.06 and a normalization of 2.46 ± 0.09 photons cm^-2 s^-1 sr^-1 keV^-1 at 3 keV (~11.6 photons cm^-2 s^-1 sr^-1 keV^-1 at 1 keV), corresponding to a 2–10 keV flux of (2.24 ± 0.16)  10^-11 erg cm^-2 s^-1 deg^-1 (90% confidence level, including the absolute flux calibration uncertainty). Our results are in excellent agreement with two of the most recent CXB measurements, performed with BeppoSAX LECS/MECS data (Vecchi et al. [CITE]) and with an independent analysis of XMM-Newton EPIC/MOS data (Lumb et al. [CITE]), providing a very strong constraint to the absolute sky surface brightness in this energy range, so far affected by an ~40% uncertainty. Our measurement implies that the fraction of CXB resolved by the recent deep X-ray observations in the 2–10 keV band is of 80 ± 7% (1σ), suggesting the existence of a new population of faint sources, largely undetected within the current sensitivity limits of the deepest X-ray surveys.